1. Field of the Invention
This invention relates to an image forming apparatus, such as a laser-beam printer or a copier, including an image bearing member, such as a photosensitive member or a dielectric member, and a charging member for charging the image bearing member, and to a process cartridge which is detachable with respect to the image forming apparatus.
2. Description of the Related Art
In many image forming apparatuses, such as copiers or the like, process units, such as an image bearing member (a photosensitive drum), a cleaning unit, a developing unit and the like, are accommodated within a cartridge, so that they are detachable with respect to the main body of the apparatus as one body in the form of a process cartridge. In such an image forming apparatus, when any of the process units within the cartridge has come to an end of its life, or any consumable material (for example, toner within the developing unit) has been used up, the entire process cartridge is exchanged for ease of maintenance.
Chargers, such as a primary charger, a transfer charger, and the like, are disposed around the photosensitive drum so as to supply various kinds of electric charges to the photosensitive drum. A noncontact-type corona charger utilizing corona discharge, a charging roller used in contact with the photosensitive drum in the form of a roller, or the like may be used as such a charger.
If a corona charger is used as the primary charger, noxious ozone is generated during corona discharge. Accordingly, an ozone filter is required within the image forming apparatus, thereby causing an increase in production costs and also an increase in the size of the image forming apparatus.
If a contact-type charger in pressure contact with the photosensitive drum, such as a charging roller, is used as a primary charger instead of the corona charger, the generation of ozone is pevented.
However, in conventional image forming apparatuses, if there is an extended time period until the process cartridge is used after it has been manufactured, or if there is an extended time period until the process cartridge is recycled after it has been used, the charging roller stays in pressure contact with the photosensitived drum at the same portion. Hence, for example, traces caused by pressure may be produced in the charging roller and the photosensitive drum, contact portions of the charging roller and the photosensitive drum may be permanently deformed due to creep, or the photosensitive drum may be contaminated with materials contained in the charging roller, thereby degrading the obtained image. The same problems will arise also when a charging blade is used as a contact-type charger.
In order to overcome the above-described problems, a technique is disclosed, for example, in U.S. Pat. No. 5,095,335, in which a charging roller provided in the main body of an image forming apparatus is made to be in contact with or separated from a photosensitive drum provided in a process cartridge linked with mounting and detaching of the process cartridge. However, since a charging roller is not provided in the process cartridge, the mechanism of mounting and detaching the process cartridge to the image forming apparatus becomes complicated.